fix(kumactl) use strings for stderr output test check

[jpeach]

Summary

If we assert that kumactl standard error is empty using a byte buffer,
then the Ginkgo error shows a hex dump, which isn't that easy to read.
Switch to checking for an empty standard error string so that Ginkgo
shows any error messages directly in the test failure.

Full changelog

N/A

Issues resolved

N/A

Documentation

N/A

Testing

• Unit tests
• E2E tests
• Manual testing on Universal
• Manual testing on Kubernetes

Backwards compatibility

• Add backport-to-stable label if the code is backwards compatible. Otherwise, list breaking changes.

[codecov-commenter]

Codecov Report

Merging #2323 (ce3475f) into master (1c6a7c2) will increase coverage by 0.39%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2323      +/-   ##
==========================================
+ Coverage   50.64%   51.04%   +0.39%     
==========================================
  Files         871      875       +4     
  Lines       40631    40664      +33     
==========================================
+ Hits        20579    20758     +179     
+ Misses      18062    17881     -181     
- Partials     1990     2025      +35     

Impacted Files	Coverage Δ
api/internal/util/proto/proto.go	62.50% <0.00%> (ø)
api/internal/util/proto/types.go	100.00% <0.00%> (ø)
...kg/plugins/resources/k8s/native/pkg/test/within.go	100.00% <0.00%> (ø)
...k8s/native/controllers/proxytemplate_controller.go	0.00% <0.00%> (ø)
pkg/core/resources/model/rest/resource.go	66.66% <0.00%> (+1.51%)	⬆️
api/mesh/v1alpha1/dataplane_helpers.go	77.84% <0.00%> (+1.70%)	⬆️
pkg/insights/resyncer.go	62.91% <0.00%> (+2.81%)	⬆️
pkg/mads/server/server.go	84.76% <0.00%> (+2.85%)	⬆️
pkg/plugins/resources/memory/store.go	79.74% <0.00%> (+4.43%)	⬆️
api/mesh/v1alpha1/dataplane_insight_helpers.go	80.24% <0.00%> (+4.93%)	⬆️
... and 10 more

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